In a known brake booster of the kind specified (DE-A 4211849) the piston is configured exclusively to transfer actuating forces and to mechanically control the valve barrel. The stopper element is secured to the piston and extends outwardly through radial recesses of the valve barrel and valve body in alignment with each other where it coacts with a stopper formed on the booster housing. The recess of the valve barrel is dimensioned so that its resting position is dictated by the valve barrel being in direct contact with the stopper element. The seal interposed between valve barrel and piston is a lip-type seal held in an annular groove of the piston and, irrespective of the position of the valve barrel with respect to the piston, is in sealing contact with a cylindrical inner wall of the valve barrel.
In this known assembly the valve barrel may be solenoid actuated to support the wanted braking effort of the driver produced by mechanical force input via the actuator element, when a sensor arranged for example on the actuator element senses that a certain actuating rate is exceeded which is interpreted as being a sign that the driver intends to initiate braking action in an emergency. Should the driver subsequently recognize that instead of emergency braking, normal braking is sufficient, he will reduce the pressure exterted on the brake pedal and thus trigger actions in the brake booster which may be solenoid monitored and taken as an indication to de-energize the solenoid and thus allow the valve barrel to return to its rest position. However, electronically monitoring the components within the booster is a problem, because it is difficult to accommodate the wiring needed for this so as not to become a nuisance whilst being liable to malfunction itself.
The invention is thus based on the object of providing simple means less liable to malfunction for discontinuing solenoid-triggered boosting of braking action in an emergency.
This object is achieved according to the invention by the features of claim 1.